Distribution line clamp force using DC bias on coil

ABSTRACT

A power distribution monitoring system is provided that can include a number of features. The system can include a plurality of monitoring devices configured to attach to individual conductors on a power grid distribution network. In some embodiments, a monitoring device is disposed on each conductor of a three-phase network and utilizes a split-core transformer to harvest energy from the conductors. The monitoring devices can be configured to harvest energy from the AC power grid and apply a DC bias to core halves of the split-core transformer to maintain a positive magnetic force between the core halves. Methods of installing and using the monitoring devices are also provided.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C. 119 of U.S.Provisional Patent Application No. 61/583,117, filed Jan. 4, 2012,titled “Distribution Line Clamp Force Using DC Bias on Coil”, whichapplication is incorporated by reference as if fully set forth herein.

INCORPORATION BY REFERENCE

All publications and patent applications mentioned in this specificationare herein incorporated by reference to the same extent as if eachindividual publication or patent application was specifically andindividually indicated to be incorporated by reference.

FIELD

The present application relates generally to distribution linemonitoring, sensor monitoring, and power harvesting.

BACKGROUND

Power harvesting using induction pick-up from the magnetic fieldsurrounding a power distribution line can be used to power distributionline monitoring sensors. Typically, the power line is routed through acurrent transformer whereby an AC signal is derived from the magneticfield induced by the AC current flow in the distribution line. The ACsignal is converted to DC as part of the power harvesting process andused to power the monitoring sensors and associated electronics. This istypically referred to as “inductive harvesting using currenttransformers.”

One method of mounting the current transformer on the distribution lineis to cut the C.T. in two, mount the halves around the uncutdistribution line, and mechanically hold the two C.T. halves together.The changing magnetic field (AC) causes the magnetic force of attractionbetween halves of a split core current transformer to alternate betweena zero force and a peak force at twice the AC line frequency. Duringoperation, the core halves need to be mechanically held together, whichcan be challenging in a hot-stick deployed sensor application.

SUMMARY OF THE DISCLOSURE

A method of monitoring a power grid distribution network is provided,comprising harvesting energy from a conductor line of the power griddistribution network with a split-core transformer of a monitoringdevice installed on the conductor line, powering the monitoring devicewith the harvested energy, and applying a DC bias to the split-coretransformer to maintain a net positive magnetic force between first andsecond core halves of the split-core transformer.

In some embodiments, the magnetic force between the first and secondcore halves never equals zero during a full AC cycle on the conductorline.

In other embodiments, the method further comprises producing the DC biaswith a circuit disposed within the monitoring device. In one embodiment,the circuit enables secondary currents flowing in secondary windings ofthe split-core transformer to have different magnitudes on eachhalf-cycle of the AC cycle.

Another embodiment comprises sensing electrical parameters of the powergrid distribution network with the monitoring device.

Another method of monitoring a power grid distribution network isprovided, comprising installing a split-core transformer of a monitoringdevice around a conductor line of the power grid distribution network,the split-core transformer having a first core half and a second corehalf, harvesting energy from the conductor line with the split-coretransformer, and during the harvesting energy step, applying a DC biasto the split-core transformer to maintain a net positive magnetic forcebetween the first and second core halves of the split-core transformer.

In some embodiments, the magnetic force between the first and secondcore halves never equals zero during a full AC cycle on the conductorline.

In other embodiments, the method further comprises producing the DC biaswith a circuit disposed within the monitoring device. In one embodiment,the circuit enables secondary currents flowing in secondary windings ofthe split-core transformer to have different magnitudes on eachhalf-cycle of the AC cycle.

Another embodiment comprises sensing electrical parameters of the powergrid distribution network with the monitoring device.

A power line monitoring device is also provided, comprising a split-corecurrent transformer comprising first and second core halves, thesplit-core transformer being configured to harvest energy from aconductor line of a power grid distribution network to power the powerline monitoring device, secondary windings disposed around at least thefirst core half of the split-core transformer, and a circuitelectrically coupled to the secondary windings, the circuit configuredto apply a DC bias to the secondary windings to maintain a net positivemagnetic force between the first and second core halves.

In some embodiments, the device further comprises sensing elementsconfigured to monitor electrical and environmental parameters of thepower grid distribution network.

In some embodiments, the circuit is configured to produce secondarycurrents in the secondary windings that have different magnitudes oneach half cycle of an AC line current of the conductor line.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features of the invention are set forth with particularity inthe claims that follow. A better understanding of the features andadvantages of the present invention will be obtained by reference to thefollowing detailed description that sets forth illustrative embodiments,in which the principles of the invention are utilized, and theaccompanying drawings of which:

FIG. 1A is a typical over-head three-phase power distribution systemutilizing a cross-bar mounted on pole for mechanical positioning of theconductors. Alternate patterns of parallel conductor routing aresometimes used. Power distribution line monitoring devices (102,104,106)are attached to the power lines typically using a standard lineman'sshotgun hotstick (106) for easy deployment with necessitating turningoff power in the lines.

FIGS. 1B and 1C show a schematic representation of a monitoring sensorin the closed (1B) and open (1C) positions. The open positionfacilitates mounting the monitoring sensor on a power line. The sensorremains on the power line in the closed (1B) position.

FIG. 2A is a schematic representation of the lower half of a powerharvesting current transformer with ferromagnetic core 204 and turns ofwire 202 (insulated copper wire is typical).

FIG. 2B shows the upper half of the power harvesting current transformerpositioned above the lower half in what would be the closed position fornormal operation. The upper and lower core halves separate with themechanics of the housing to facilitate mounting the core on a powerline.

FIG. 3A is a time graph of the magnetic force (two lines) between eachface of the upper and lower permeable core halves resulting from theinduced magnetic flux (one line) by the power line current and the ACcurrents flowing in the winding wire.

FIG. 3B is a time graph of the magnetic force (two lines) between eachface of the upper and lower permeable core halves resulting from theinduced magnetic flux (one line) by the power line current and the ACand DC currents flowing in the winding wire. The DC component of thecurrent flowing in the winding wire causes an asymmetrical attractiveforce in the core halves.

FIG. 3C is a time graph of the magnetic force (two lines) between eachface of the upper and lower permeable core halves resulting from theinduced magnetic flux (one line) by the power line current and the ACand DC currents flowing in the winding wire. The DC component of thecurrent flowing in the winding wire causes an asymmetrical attractiveforce in the core halves. The DC component raises the flux density highenough that the magnetic flux density saturation level of the materialis reached and the flux density and force can raise no further.

FIG. 3D illustrates the line current and the resultant core fluxdensity.

FIG. 4 shows a configuration for energy harvesting using a split-corecurrent transformer and an AC-to-DC convertor using a rectifier bridgecircuit and capacitor.

FIG. 5 illustrates a configuration for energy harvesting and AC-to-DCconversion using a saturable reactor circuit architecture to generate anet attractive force on the split-core current transformer.

DETAILED DESCRIPTION

Power line monitoring devices and systems described herein areconfigured to measure the currents and voltages of power griddistribution networks. Referring to FIG. 1A, monitoring system 100comprises monitoring devices 102, 104, and 106 mounted to power lines108, 110, and 112, respectively, of power distribution network 114. Thepower distribution network can be a three phase AC network, oralternatively, a single-phase network, for example. The powerdistribution network can be any type of network, such as a 60 Hz NorthAmerican network, or alternatively, a 50 Hz network such as is found inEurope and Asia, for example. Power distribution networks, such as inthe United States, typically operate at a medium voltage (e.g., 4 kV to46 kV or higher) to reduce the energy lost during transmission over longdistances. The monitoring devices can also be used on high voltage“transmission lines” that operate at voltages higher than 46 kV.

Monitoring devices 102, 104, and 106 can be mounted on each power lineof a three-phase network, as shown, and can be configured to monitor,among other things, current flow in the power line and currentwaveforms, conductor temperatures, ambient temperatures, vibration, windspeed and monitoring device system diagnostics. In some embodiments, afourth sensor can be mounted on the ground line near the three phaselines. In additional embodiments, multiple sensors can be used on asingle phase line. The monitoring devices can be mounted quickly andeasily via a hot-stick 116, and can harvest energy from the power linesfor operation with or without additional supplemental power (e.g.,include batteries or solar panels). The monitoring devices can furtherinclude wireless transmission and receiving capabilities forcommunication with a central server and for communications between eachmonitoring device. Installation of a three monitoring device array canbe placed and configured by a single linesman with a hot-stick and abucket truck in less than 20 minutes. Monitoring device communicationwith the installation crew can be enabled during the installationprocess to provide immediate verification of successful installation.FIG. 18 illustrates a monitoring device in a closed/clampedconfiguration, and FIG. 1C shows the monitoring device in anopened/installation configuration. It should be understood that thedevice is opened into the installation configuration during installationon power lines, then closed around the line in the clamped configurationprior to operation.

Furthermore, monitoring devices 102, 104, and 106 are configured tomeasure the electric field surrounding the power lines, to record andanalyze event/fault signatures, and to classify event waveforms. Currentand electric field waveform signatures can be monitored and cataloguedby the monitoring devices to build a comprehensive database of events,causes, and remedial actions. In some embodiments, an applicationexecuted on a central server can provide waveform and event signaturecataloguing and profiling for access by the monitoring devices and byutility companies. This system can provide fault localizationinformation with remedial action recommendations to utility companies,pre-emptive equipment failure alerts, and assist in power qualitymanagement of the distribution grid.

Monitoring devices 102, 104, and 106 can comprise sensing elements, apower supply, a battery, a microprocessor board, and high poweredcommunication systems (not shown) disposed within a robust mechanicalhousing designed for severe service conditions. The monitoring devicesare configured to withstand temperatures ranging from −40 to +85 C., EMIand ESD immunity, current and voltage impulse resistance, driving rainand precipitation and salt fog survival. A typical embodiment of themonitoring devices is configured to operate continuously on power linescarrying up to 800 A_(RMS) operating current with full functionality.Full functionality is also maintained during line fault current eventsup to 10 kA_(RMS) and of limited time duration.

The monitoring devices can be configured to communicate wirelesslythrough a distribution network, such as through the Silver SpringNetwork, to the power utilities sensor control and distributionautomation (SCADA) system. In some embodiments, the monitoring devicesoperate at 1 watt with a custom designed omni-directional antenna. Whenmounted to typical power grid distribution networks, the monitoringdevices are located approximately 30 feet above ground level andtypically above tree tops, providing for a very substantial effectiverange of communication. In addition to two-way network communicationsfor data packets and setting operational setpoints, the monitoringdevices can be configured for wireless device firmware upgrades for longterm functionality.

The monitoring devices described herein can also include powerharvesting systems configured to convert the changing magnetic fieldsurrounding the distribution lines into current and/or voltage that canbe rectified into DC current and used to power the monitoring devices.FIGS. 2A-2B illustrate one embodiment of a power harvesting system 200,which can be included in the monitoring devices 102, 104, and 106 ofFIGS. 1A-1C. In some embodiments, the power harvesting system ispositioned in the monitoring devices so as to surround the power lineswhen the monitoring devices are installed.

Referring to FIG. 2A, power harvesting system 200 can include harvestingcore secondary windings 202 around a first half 204 of a split corecurrent transformer. The current induced in the harvesting coresecondary windings can be consumed in an asymmetrical manner regardingthe phase and thus produce a net DC biased load current. The DC biasedcurrent in the harvesting core windings directly produces a bias in themagnetic flux density that in turn makes a net bias in the attractivemagnetic force between the core halves. The bias in the load current canbe made high enough to maintain a net positive magnetic force (shown asMag-Force in FIG. 2A) on the core halves even as the AC line currentcontinues to produce an oscillatory magnetic flux. The DC bias currentscheme can be implemented using active non-linear circuitry of any of atype of analog, mixed analog and digital, or microprocessor-based type,some form of energy storage in the circuitry of the monitoring device,such as a battery, capacitor, inductor, etc. Holding the core halvestogether with a magnetic force advantageously relieves the amount ofmechanical force required to hold the core halves together.

The DC bias induced in the core winding is nominally continuous suchthat some net force of attraction is always induced between the corehalves. The power line induced AC component will create a time varyingforce that is cyclically greater than or equal to the DC bias inducedforce. Maintaining some amount of DC bias force substantially reduces oreliminates the core vibrational noise, keeps the core faces held incontact to reduce slippage, and avoids the generation of inter-gapparticulate from the core faces chattering together. The DC bias currentnecessary to provide these benefits is a function of the number ofwinding turns, operating current range of the line current, and otherfactors. A DC bias current of single digit milliamperes to severalhundred millamperes can be used, for example.

As described above, in a conventional system without a DC bias, thechanging magnetic field (AC) on the power lines can cause the magneticforce of attraction between halves of a split core current transformerto alternate between a zero force and a peak force at twice the AC linefrequency. FIG. 2B illustrates the second half 206 of the split corecurrent transformer being held in place against the first half 204 withthe magnetic force induced by the DC bias. The core halves 202 and 204are forcefully attracted together by the magnetic field in the corehalves. A DC bias current imposed on the core winding wires (A-B) cankeep the magnetic force in a net-positive direction so that the halvesare always being forced together.

Applying a DC bias to the core windings can have the further benefit ofreducing vibration noise caused by the energy harvesting system. Withouta DC bias, the magnetic force between core halves goes to zero on eachhalf cycle (e.g., 120 times per second on a 60 Hz line, as shown in FIG.3A). The magnetostriction of the core material and the mechanics of thedesign can cause mechanical energy storage as compression displacementdue to the magnetic force field. This energy can be released when themagnetic field or flux goes near zero and the magnetostriction forcelikewise goes to near zero force. The core halves relax and maymechanically separate from another slightly. This relaxation andseparation can be brought back together on the next half cycle of the ACcurrent, creating an audible sound that manifests as a 120 Hz buzzingnoise. Thus, in some embodiments, applying a DC bias to one of the corehalves can eliminate the periodic separation of the halves, therebyeliminating vibration noise in the monitoring devices when mounted onpower lines.

Additionally, applying a DC bias to the core halves results inadditional control gained for the removal of the monitoring devices frompower lines. In a monitoring device without a DC bias, the removalprocess requires overcoming the magnetic attractive force to separatethe core halves. This force can be substantial and require excessivemechanical design, adding cost and weight to the device. However, in oneembodiment, the DC bias can be lowered to levels that allow for easyremoval.

FIGS. 3A-3D illustrate the magnetic flux density in the core material ofthe split-core transformer resulting from the AC line current. Themagnetic flux density is closely proportional to the line current asderived from Ampere's Law for magnetic fields up to the magneticsaturation level of the core material. For a toroidal shaped currenttransformer core the flux density B is closely approximated by where:B=μ ₀ nI/L=μ ₀ nI/2πr

-   -   where:    -   “B” is the flux density, in Tesla    -   “μ₀” is the permeability of space, equals 4π×10−7 T·m/A    -   “n” is the number of turns of winding wire, equals 1 for primary        conductor    -   “I” is the current in the winding wire, in Amperes    -   “L” is the circumferential length of toroidal core centerline,        in m²    -   “r” is the centerline radius, in m

The mechanical force between two nearby magnetized surfaces of area “A”can be calculated knowing the magnetic flux density “B” transecting thesurfaces. In a split-core current transformer the total magnetic fluxdensity in the core results from the superposition of the flux densitiesof the windings; the first being the single-turn primary line conductor,the second being the energy harvesting secondary winding. The netmagnetic force between the split-core halves at the cut faces can becalculated with the following equation. The equation is for cases inwhich the effect of magnetic flux fringing is negligible and the volumeof the air gap is much smaller than that of the magnetized corematerial.Force=(μ0H2A)/2=B2A/(2μ₀)

-   -   Where:    -   “A” is the area of each cut core surface, in m²    -   “H” is the magnetizing field, in A/m.

FIG. 3A illustrates the magnetic flux density and magnetic force in asplit core current transformer resulting from 60 Hz line current. FIG.3B illustrates the magnetic force in a split core current transformerresulting from line current plus DC bias imposed on the secondaryharvest output winding. As shown in FIG. 3A, the magnetic force in atraditional split core current transformer power harvesting design goesto zero as the current on the conductor transitions from positive tonegative and negative to positive. In contrast, in FIG. 3B, applying aDC bias to the split core current transformer results in a correspondingbias in the magnetic flux density causing there to always be a positivemagnetic force across the transformer gaps, even as the alternatingcurrent transitions from positive to negative. FIG. 3C illustrates theflux density reaching the core material's maximum saturation fluxdensity of 1.7 Tesla and the magnetic force limiting incurred by theflux saturation.

FIG. 3D illustrates the line current and the resultant core fluxdensity.

Referring to FIG. 4, one embodiment to harvest energy from the magneticfield generated by current flow in a distribution line conductor is touse a split-core current transformer CT1. The primary side of CT1 is thesingle distribution line conductor passing through the core. Thesecondary side of CT1 is formed by turns of a smaller gauge wire woundonto the CT core usually with many turns; number of turns being anywherefrom one to thousands depending on the application with 100 to 1000being a common range. An alternating current (AC) flowing in the primaryconductor establishes an alternating magnetic flux in the permeable corematerial. The mutual coupling of the core to the secondary windingestablishes a current flow in the secondary. The current flowing in thesecondary is proportional to the current in the primary with theproportionality coefficient equal to the ratio of primary turns tosecondary turns. The energy that can be transferred from primary currentto secondary current is limited in amount by the limited permeability ofthe core material. Generally a core material with high permeability isselected for this application to maximize the amount of energytransferrable. When the core reaches its maximum magnetic flux density,called saturation, the transfer mechanism ceases to operate and no moreenergy will transfer until the core is unsaturated. This unsaturationcan happen when the primary current reverses direction or ceasesflowing.

The magnetic flux density in the permeable split-core establishes anattractive force between the core halves at the adjoining faces at eachsplit. The force generated across the gap is attractive for each halfcycle of alternating current. Those in the industry commonly understandthis property. While on each half cycle there is an attractive forcewhen the magnetic flux is established, there is also two moments percycle when the magnetic flux density in the core is equal to zero. Thesetwo points occur when the magnetic flux is in transition from onedirection to the other. An undesirable consequence of these zero-levelmagnetic flux densities is that a mechanical relaxation occurs when theattractive force goes to zero resulting in audible hum or buzz in CT1assembly. The audible noise is roughly proportional to the magnitude ofthe current flowing in the primary; more primary current generallyresults in more audible noise. Audible noise is an undesirable sideeffect of the harvesting process.

Generally a high force is applied by external means in an attempt toovercome the magnetic relaxation forces and thus prevent audible noise.However, in a distribution grid power harvesting application where thedevice is installed using the utility industry standard hot-stick, thereis great difficulty in designing a mechanism to apply the necessarymechanical force at reasonable cost and weight.

The full-wave bridge rectifier circuit formed by rectifiers D1-D4 andcapacitor C1 can be used to create a DC voltage from the AC currentflowing in the secondary windings. The DC voltage is established oncapacitor C1 and can used to power measurement and communicationscircuits and systems of the monitoring devices described above.

A brief description of the circuit operation of FIG. 4 is given. ACcurrent flow, Ic, in the primary of CT1 transfers an alternating currentinto the secondary winding of CT1 by the mutual inductance of the splithigh permeability core. During one half cycle of primary current Ic,flowing in the direction of the arrow shown, develops current Ict1 inthe secondary, in one particular direction (for example as shown byarrow). The secondary current Isec travels from J2 through rectifier D2,capacitor C1 and load, rectifier D3, and back through secondary windingof CT1 at J1.

During the second half-cycle of the primary current, when the primarycurrent flows in the reverse direction to the arrow shown, the secondarycurrent Isec flows from J1 through rectifier D1, capacitor C1 and load,rectifier D4, and back through secondary winding CT1 at J2. In this wayboth the positive and negative half cycles of line current Ic producethe same direct current direction through the load and output capacitorC1. This is the commonly understood principle of the full-wave rectifiercircuit.

A key point of the basic full-wave rectifier circuit is that themagnitudes of the currents flowing in CT1 secondary during each halfcycle are the same. There is no distinction by the load or outputcapacitor C1 whether the secondary current is flowing in the positive ornegative direction. As the net magnetic flux produced by CT1 primary andsecondary currents alternates from positive to negative, and negative topositive, there results in two points in each cycle where the magneticflux density in CT1 is zero. The two points at which the magnetic fluxis zero results in two moments of zero magnetic force between the corehalves.

Those that practice power supply design will recognize theover-simplification of the design to highlight the architecture of thecircuit. Other circuit components are typically included to reduceelectromagnetic interference (EMI), provide over-current andover-voltage protection, connect electrical elements together andsupport heat dissipation. Rectifiers can be one or a combination ofseveral types including, but not exclusive of others, silicon PNjunction diodes, Schottky diodes, MOSFETs or bipolar transistorsactively driven, etc. Energy storage for operating circuits can beaccomplished by capacitors, batteries, super-capacitors, etc.

Referring to FIG. 5, the basic full-wave rectifier circuit is modifiedto enable the secondary currents Isec to have different magnitudes oneach half cycle of the line current. The result of enabling thesecondary currents to have different magnitudes on each half cycle ofthe line current is that there no longer exists two points during eachcycle where the magnetic flux is zero, and therefore, there are nolonger moments of zero magnetic force between the core halves. Onemethod of accomplishing this is by breaking the rectifier bridge circuitat cathodes of D2 and D1 and adding additional circuit elements C2,circuit block X1, R1, and D5. Circuit block X1 can be derived fromactive components and powered from energy stored on C2, C1, or on abattery or super-capacitor charged from the rectifier circuit. Thus, thecircuit shown in FIG. 5 can be implemented in a power line sensordescribed herein to provide a DC bias to the split-core transformer tomaintain a net positive or attractive magnetic force between the firstand second core halves of the split-core transformer.

The capacitor C1 in FIGS. 4 and 5 can be the energy storage element. C1will charge up and hold voltage on each half cycle of the line current.This voltage can then used to power circuits (measurements,communication radio, etc) within the power line sensor. The specialcircuit X1 added in FIG. 5 can be powered from the energy stored in C1or in C2 (as shown). Circuit X1 is responsible for creating the DC biascurrent in the secondary winding. Without circuit X1 (as in FIG. 4) thecurrent in secondary winding is the same magnitude for a positivehalf-cycle and a negative half-cycle, just opposite direction. To changedirection means the current had to pass thru zero (+ to 0 to − to zeroto + . . . etc).

As for additional details pertinent to the present invention, materialsand manufacturing techniques may be employed as within the level ofthose with skill in the relevant art. The same may hold true withrespect to method-based aspects of the invention in terms of additionalacts commonly or logically employed. Also, it is contemplated that anyoptional feature of the inventive variations described may be set forthand claimed independently, or in combination with any one or more of thefeatures described herein. Likewise, reference to a singular item,includes the possibility that there are plural of the same itemspresent. More specifically, as used herein and in the appended claims,the singular forms “a,” “and,” “said,” and “the” include pluralreferents unless the context clearly dictates otherwise. It is furthernoted that the claims may be drafted to exclude any optional element. Assuch, this statement is intended to serve as antecedent basis for use ofsuch exclusive terminology as “solely,” “only” and the like inconnection with the recitation of claim elements, or use of a “negative”limitation. Unless defined otherwise herein, all technical andscientific terms used herein have the same meaning as commonlyunderstood by one of ordinary skill in the art to which this inventionbelongs. The breadth of the present invention is not to be limited bythe subject specification, but rather only by the plain meaning of theclaim terms employed.

What is claimed is:
 1. A method of monitoring a power grid distributionnetwork, comprising the steps of: harvesting energy from a conductorline of the power grid distribution network with a split-coretransformer of a monitoring device installed on the conductor line;powering the monitoring device with the harvested energy; and applying aDC bias to the split-core transformer to maintain a net positivemagnetic force between first and second core halves of the split-coretransformer.
 2. The method of claim 1 wherein the magnetic force betweenthe first and second core halves never equals zero during a full ACcycle on the conductor line.
 3. The method of claim 2 further comprisingproducing the DC bias with a circuit disposed within the monitoringdevice.
 4. The method of claim 3 wherein the circuit enables secondarycurrents flowing in secondary windings of the split-core transformer tohave different magnitudes on each half-cycle of the AC cycle.
 5. Themethod of claim 1 further comprising sensing electrical parameters ofthe power grid distribution network with the monitoring device.
 6. Amethod of monitoring a power grid distribution network, comprising thesteps of: installing a split-core transformer of a monitoring devicearound a conductor line of the power grid distribution network, thesplit-core transformer having a first core half and a second core half;harvesting energy from the conductor line with the split-coretransformer; and during the harvesting energy step, applying a DC biasto the split-core transformer to maintain a net positive magnetic forcebetween the first and second core halves of the split-core transformer.7. The method of claim 6 wherein the magnetic force between the firstand second core halves never equals zero during a full AC cycle on theconductor line.
 8. The method of claim 7 further comprising producingthe DC bias with a circuit disposed within the monitoring device.
 9. Themethod of claim 8 wherein the circuit enables secondary currents flowingin secondary windings of the split-core transformer to have differentmagnitudes on each half-cycle of the AC cycle.
 10. The method of claim 6wherein the power grid distribution network comprises an AC power griddistribution network.
 11. The method of claim 6 further comprisingsensing electrical parameters of the power grid distribution networkwith the monitoring device.
 12. A power line monitoring device,comprising: a split-core current transformer comprising first and secondcore halves, the split-core transformer being configured to harvestenergy from a conductor line of a power grid distribution network topower the power line monitoring device; secondary windings disposedaround at least the first core half of the split-core transformer; and acircuit electrically coupled to the secondary windings, the circuitconfigured to apply a DC bias to the secondary windings to maintain anet positive magnetic force between the first and second core halves.13. The device of claim 12 further comprising sensing elementsconfigured to monitor electrical and environmental parameters of thepower grid distribution network.
 14. The device of claim 12 wherein thecircuit is configured to produce secondary currents in the secondarywindings that have different magnitudes on each half cycle of an AC linecurrent of the conductor line.